When a file is "deleted" it is not truely deleted, of course. I was on windows 8.1.

Anyway, I accidentally screwed something up which led to more screw-ups which led to an unbootable hard drive and the "deletion" of all my files (I'm sure those files are not truly erased). Fortunately, I had a Ubuntu cd lying around to install a new OS. Upon installation, I used the native disk encryption and chose a 20+ character password. I did not select "overwrite hard drive" because I was afraid it would take too long. It didn't take too long to install and restart, so I'm wondering whether a secure encryption algorithm was used (Trucrypt takes much longer). Most importantly, I had about 200GB+ data on my hard drive before this and I am wondering if those "deleted" files are also encrypted. In other words are those "deleted" files safe even though they were from a bygone era and different OS. I mean how could it have encrypted all those hidden files so fast?

I immediately used bleach bit to delete all "free space," but IDK man you can't be sure.

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Full disk encryption

If you use full disk encryption and you wait for it to fill the entire volume with random data (which may take hours on a large drive) then yes, previous deleted files will be gone and replaced with random data - for that partition. Data may still remain in other partitions or unpartitioned space if any.

Future files you delete will be recoverable only with the same decryption key.

If it didn't take hours to create the encrypted volume then most likely it didn't fill the volume with random data and previously deleted files may still be there.

In this case using other software to fill all unused space with random data will suffice, especially if it does "free space" and "cluster tips". Doing this with zeroes or semi-random data is not recommended as it can aid in cryptanalysis, but at least your previously deleted files will be gone.

Note that for modern SSDs with wear-levelling, wiping/writing random data to the entire disk may not entirely wipe all old data as the SSD keeps additional data blocks and swaps data around.

File based encryption

File based encryption, such as when you encrypt your home directory only, only operates per-file. Old deleted files that weren't encrypted won't be touched. Again you can improve privacy by wiping free space using other software.

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